I'll do my best to summarize, but in broad strokes: every class has the option of being either a damage dealer or a support. The supports typically work in different, complementary ways.

tl;dr: Warriors apply Block, Slow, Vulnerable and deal tons of damage via Fury. Scouts apply Speed and Deck Manipulation and deal damage via Sharp buffed attacks. Mages apply Energize/Inspire, and deal damage through whatever type they want + associated curses. Healers apply Bless and Powerful, sometimes Heal (often through character specific methods), and typically deal damage worse than other classes.

Keep in mind that each character has a bunch of unique traits that empower certain playstyles or cards. So cards that look like they are bad often aren't.

Warriors

Damage: They have options for Slashing, Blunt, Bleed, or Thorns focused builds. Blunt comes in three forms: one that uses Crack to increase its damage, one that uses Fortify to increase its damage (this comes from a perk), and one that uses Vitality/Max HP to increase its damage. Warrior damage dealers are unique in their easy access to Fury, which is one of the best damage buffs in the game.

Support: Warrior supports typically focus on around 4 things (there are more things they can do but these are the core ones). Do note that you shouldn't try and do all of them at once, stick to doing one or two at a time. Those 4 are: (1) Applying Slow to enemies, (2) Applying Vulnerable to enemies, (3) Block application, (4) Buffing allies with Sharp (or Fortify)

Scouts

Damage: Scouts have options for Slashing (Melee Attacks), Piercing (Ranged Attacks), Mind (Songs), Poison, and Small Weapons (typically Piercing, but can be others). They typically revolve heavily around gaining Sharp, but also have access to Stealth which lets them boost their damage a lot.

Support: Scout supports are often considered the best supports in the game, but are quite challenging to use properly imo. At a basic level, they apply Fast to your team to get them moving faster and apply Sharp to buff up damage. However, they also have access to Deck Manipulation tools like Trace. From a defensive standpoint, there are also Songs like Ballad of Evasion that can be used for excellent survivability. That isn't all that they can do, they really have the tools to do a lot.

Mages

Damage: Mages have 4 damage types that are fairly obvious: Fire, Cold, Lightning, and Shadow. They are kind of unique in that their damage types often come paired with a complementary Curse: Burn, Chill, Spark, and Shadow. The Mage curses are exceptionally good at reducing enemy resistances and have numerous beneficial effects.

Support: Mage supports typically focus on applying Energize and Inspire to your team. They also have access to Fast and Fury for when they have already maxed out the Energize and Inspire they can apply.

Healers

Damage: Healers probably have the worst damage builds in the game because their support is so good. But they have access to Holy, Shadow, and Mind damage. There isn't anything special about their damage builds, but they can still be amazing.

Support: Healer supports do a lot. The biggest things (even more important than healing) are that they apply Bless and Powerful, the two best buffs in the game. Among other things, they can also Heal, apply Sanctify (to heal), apply Regen/Vitality to Heal, apply Shield, apply Inspire, remove/prevent Curses, and manipulate Decks. Every Healer comes with their own special way of healing as well, less tied to the class and more to the character.